Astrocytes support neurons and map out nonoverlapping domains in grey matter of the brain. The astrocytes of the glia limitans, however, do overlap. Using ultrastructural tools and immunogold histochemistry a subtype of astrocyte able to assemble large lamellar stacks was investigated at the ventral surface of the brain near the hypothalamus. Lamellar stacks were subsequently discovered also in the internal glia limitans of the epithalamus. Circular lamellar stacks containing AQP4 water channels surround neuronal processes, and might serve as osmosensors. The lamellar stacks are well-organized and can form over 100 membrane layers between neuropil and the basal membrane, but a barrier function is not obvious from the noncontinuous character of the stacks along the glia limitans.
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